Photoelectric tube



PHOTO ELECTRIC TUBE Filed May 31, 1930 INVENTOR FRIKIIZ SCHROTER TT iQNE n o I Y Patented Mar. 5, 1935 PATENT.- OFFICE ,rnoronmormc TUBE Fritz Schriiter, Berlin, Germany, assignor to Telefunken Gesellschaft fiir' Drahtlose Telegraphie m. b. H., Berlin, Germany, a corporation of Germany Application May 31. 1930, Serial No. 458,059

. In Germany June 6, 1929 "-2 Claims. (01. 250-27.5)

In all technical uses of photoelectric tube actions it is desirable that the electric current of the photoelectric tube which is caused to flow by the action of incident or diffusely reflected 5 light should be as large as possible. In the case of vacuum tubes, the said current is a pure electronic current. The incident light causes the issuance of electrons from the surface layer or a layer adjacent to the surface of the photo- 10 electrically active material while incidentally overcoming a certain resistance known as the escape Work or work function, of which terms the former will be used throughout this description. The number or volume of saidelec- 15 trons emitted is proportional to the intensity of the light, whiletheir maximum speed is a function of the wave-length of the light. The energy supplied to the electrons from the light and therefore their initial velocity grows with increasing 20 frequency of the light. However, inasmuch as each electron must first be furnished with a certain minimum amount of energy in order that it may beenabled to escape from the surface layer of the photoelectrically active material, 25 there exists what is known as the limital wavelength of the light peculiar to the material so that beyond this critical value, in the direction of greater wave-lengths, no more photoelectric effect may arise. By the use of the term limital 30 wave-length applicant is intending-to designate the wave-length of that light whose energy is equivalent to the escape work or work function hereinabove mentioned and, therefore, refers to the limiting light values for producing the effects desired.

In the case of gaseous tubes, in the presence of a sufficiently high supplementary potential, the said primary electron current, in addition, will cause the flow of a far heavier secondary ionic current whereby the effectiveness of the photoelectric tube is greatly enhanced.

Now, the purpose of the present invention is, by the adoption of ways and means to be described in more detail further below, for example, to improve or increase from photoelectric tubes the yield of current by raising the primary electron current, and, in the case of gaseous tubes, also the aggregate current consisting of the electron and the ion currents by reason of a diminishing of the so-called escape work. Simultaneously therewith, a shift in the limital wave-length towards greater wave-lengths may also be obtained. From this shift, among the advantages which may be obtained are, for example, the following:-

(l) A great number of substances whose limital wave-length falls well inside the short-wave range become thus photoelectrically useful in practice; A

(2) Other substances which, to be sure, 'exhibit a photoelectric effect in the white range, result in an increased yield of current seeing that the portion of the white light possessing greater wave-length is also rendered active; and,

(3) By rendering the photoelectric material more sensitive or responsive in the direction of the longer waves, a responsiveness heretofore unknown is insured for certain spectural ranges and purposes such as are required for the picture-telegraphic transmission ofmulti-color copies. H

The invention as herein set forth' has been schematically illustrated by'the accompanying drawing, of which Fig. 1 represents a" prefer-red form and discloses a means by which a photoelectric cathode may be heated; and,

Fig. 2 discloses a modification of the invention wherein the photoelectric material is arranged in a predetermined manner as asupport surface.

Among the measuresand means that maybe mentioned to insure a decrease in the escape work of the electrons as they issue from the photoelectric material, as will become apparent from reference to the drawing, the following may be considered useful according to this invention:

1. Changes in the surface film of the photoelectrically active material-by chemical actions or by the physical or mechanical addition of surface layers of an extraneous substance, it having to be noted that the latter need not necessarily be photoelectrically active. It is known in the prior art, and, in fact, it has been customary with potassium tubes to hydride the potassium cathode superficially, or, in other 40.

words, to cover it with alayer of potassium hydride. While this process merely serves the purpose of insuring a more perfect energy absorption of the light, according to this invention a" layer of a substance is to be applied which, as is usually done, for instance, with the filaments of thermionic tubes, tends to diminish the "escape work to be overcome by the electrons as they escape therefrom. The substances which may be considered for this purpose are, e. g., the oxides of alkali-earth metals or else a thin barium or thorium film. The thickness of the film need to be only a few atoms diameter so that the film itself will still be sufiiciently permeable for the light; 66

2. Heating of the photoelectrically active material to a constant though sufficiently high temperature. By heating such a pure or a thoriated conductor, or a conductor coated as mentioned above in 1, with a layer of baryta, etc., an active thermal movement of the negative electricity carriers 'is occasioned; and if the temperature is adequately high, this results in voluntary or spontaneous escape or evaporation of the electrons from the material, as by the Richardson effect. If the cathode is illuminated zor irradiated at the same time, this results in a further growth in electron emission inasmuch as such additional energy supply enables "a stillgr'eater number of such electrons to escape as previously or priorly possessed only velocity which was inadequate to cause them to leave'thesurface-layer of the cathode. The purely thermo-electric current also flowing in the dark state of the photoelectric tube, since it is constant, can, if desired, by suitable circuit. means be separated :from the component or part due to the illumination only. Theheating of the photoelectric cathode is effected most suitably by electrical means, although this is not essential. The'catho'de itself may, be traversed by the :heating current, or else the heater body may be separately disposed, heat being thus supplied therefrom to the :photoelectricdayer-by virtueof conduction or radiation; It is an embodiment of this kind which has been shown by-Fig. 1, wherein the .photoelectrically activecathode (l) is applied upon a hollow body (2) which is entirely' or'pa'rtly closed and consists-of :electrically insulating material. Thebjody (2) is their internally heated-by means of a heater wire (3) connected with an external source of current (not shown) ,The' third consideration may be that of influencin'g. the surface -of the .photoelectrically active material by an electric-field. The issuance of photoelectricelectrons/from the surface of the conductor can -be-- facilitated by causing a suitably acting electric field to affect'th-e lattice of atoms of the photoelectrica-llyactive material in thesurface:sothatittenda-as-it were, to pull out theelectrons therefrom. It is advantageous for this purpose to bringas high afield density as possible to act upon the surface layer. Such a specialincrease of therelatively low-field density due to the potential applied thereto normally in the operation of a photoelectric tube, according to this invenion, can be-xeffecte'd by providing the surface of the cathode 1 with peaks, points or edges 5 of small radius of curvature as illustrated in Fig. 2. Instead of marked points or edges shown by Fig. 2, recourse could also be had to a more or less regular toughening of the cathode surface.

' Another chance of causing a high field density (intensity) to act upon the photoelectrically active material would consist in applying upon the said surface a thin coat of an electric semiconductor or poor conductor. In such a layer even a relatively feeble current will cause a high fall of potential, or, in other words, a high field intensity. What has further a favorable infiuence upon the electron yield in connection with such an arrangement is the circumstance that an outside electric Yfiel'd will penetrate more deeply into such a poor conductor than into a nearly perfectconductonand also that electron layers located' at great depths are thus made accessible to the-influence. It may be of advantage also to use an alternating current of low or of higher frequency to act as 'the anode potential in the operation-of-such aaphotoelectric tube. The cacapacity 'or wattless icurrent flowing through such a tube which inside thelayer of the poorconductor may partly continue as aconduction current will cause therein thepermanentmaintenance of the requisite fall of :potential.

.Fromthe above description it is apparent that many modifications of the method and means herein 'disclosed may be made, and I, therefore, believe myself to beentitled to make and use any andzall of such modifications asfairly fall within the spirit andscope of the hereinafter appended claims.

Having now described my invention, what I claim and desire to secure by Letters Patent is the following:

1. A photoelectric device comprising a hollow insulating support member, a coating of light sensitive electron: emitting material supported on one outer surface of the hollow insulating support member, and a filamentary heating unit contained within .thehollow insulating support member and covering an area substantially co-extensive with-thelight sensitive material.

2.. A photoelectricdevice comprising an insulating membena photoelectric coating positioned upon one surface of theinsulating member, and a filamentary. heating unit supported by the insulatingmember and covering an area substantiallyco-extensive with the light sensitive material and separated thereby from the electron emitting coating.

FRITZ SCHROTER. 

